Field of the Invention
The present invention concerns apparatus for a drill for down-the-hole drilling and the installation of a lining pipe in rock or soil layers according to the introduction to claim 1.
Description of the Related Art
Drills are used in prior art drill arrangements for the installation of a lining pipe, i.e. in which a lining pipe is to be left permanently in a borehole after, for example, drilling in loose rock, or in which fluids such as water or oil are to be led into the pipe, that demonstrate a central pilot drill bit that is intended to be mounted in a chuck in a down-the-hole drill using a shaft or a neck, from which impacts are transferred to the pilot bit. A control means guides the drill and the lining pipe relative to each other such that the drill can be freely rotated relative to the lining pipe. A coupling arrangement, normally in the form of a bayonet coupling, is located between the drill and the control means, which coupling arrangement when in its free condition allows the drill to be drawn back through the lining pipe together with the down-the-hole drill. The drill is intended to drill a borehole that allows the lining pipe to accompany it into the borehole. A casing shoe, which has been welded at a forward end of the lining pipe, ensures that the lining pipe is driven into the borehole together with the drill and transfers impacts from the drill to the lining pipe. The drill has internal flushing passages for the supply of flushing agent, and it has evacuation passages for the removal of drilling cuttings together with the flushing agent. Drilling takes place through a combination of impacts and rotational movement.
The transfer of impacts to the lining pipe takes place in prior art drill arrangements through the casing shoe through a forward impact surface that is a part of the drill bit acting on a rear impact surface of the casing shoe and initiating the casing shoe in this way into intermittent, axial impact motion, which is in turn transferred to the lining pipe. One problem with this design is that the output power of the hammer that is a part of the impact mechanism must be limited such that the impact energy is not sufficiently great that the welded join between the casing shoe and the lining pipe breaks. The welded join between the said parts that transfer impact energy thus constitutes a weak point. Even if the weld is of high quality, the impact energy must normally be limited when installing a lining pipe. As a consequence of the low power of the impact mechanism, the desired drilling rate is not obtained, and thus also the total capacity of the equipment used to install a lining pipe is limited.
Furthermore, if the force of feeding is too low, also the problem that the drill bits become polished arises, which means that they soon loose their cutting capacity. The drill bit may in the worst case be destroyed due to the overheating that arises. It should be realised that the possibilities for the operator to observe a broken welded join between the casing shoe and the lining pipe or a reduced drilling rate due to the loss of cutting capacity of the drill bit are limited, and that repairs to the equipment in question are both time-consuming and expensive. There is, thus, a desire to make it possible to drive this type of drill arrangement with a considerably higher hammer power than previously, not only in order to obtain an increased drilling rate but also to reduce the risk of polishing of the drill bit arising.
Drills are known from WO 9934087 A1 and US 2004/0104050 A1 that drive a lining pipe into a hole through the transfer of direct impacts from pilot bit to the lining pipe through a casing shoe. A drill is known from DE 4000691 A1 that presses a lining pipe into a borehole through the interaction between a casing shoe and a stationary part of the drill, which parts cannot be rotated at their opposing contact surfaces.